1. Field of the Invention
The present invention relates to an improved waveguide provided with a double disk window assembly having microwave transmission dielectric disks.
2. Description of the Related Art
In some cases, a circular microwave transmission waveguide employs a microwave transmission window assembly at a predetermined position, so as to keep a high degree of vacuum in the waveguide. The microwave transmission window assembly comprises a pair of dielectric disks spaced from each other by a predetermined short distance, and a coolant is made to flow in the gap between the dielectric disks, for the cooling of the disk window assembly. If a waveguide employs a double faced disk cooled window assembly having such dielectric disks its passband is widened and its window cooling efficiency is increased, so that it becomes suitable for use in a microwave apparatus which is operated at a comparatively high operating frequency and requires a transmission line for high power. Such a waveguide is disclosed, for example in U.S. Pat. Nos. 3,110,000, 3,324,427, 4,286,240, 4,371,854 and 4,620,170, and also in Japanese Utility Model Disclosure No. 60-17002. A waveguide having the above microwave transmission window assembly is employed, for example in the output section of a microwave electron tube (such as a klystron, a traveling wave tube, and a gyrotron or the microwave transmission line of a particle accelerator.
To increase the operating frequency of a wave guide, the double faced disk cooled window assembly of that waveguide has to employ thin dielectric disks and the distance between the dielectric disks has to be shortened. Further, the flow rate of the coolant has to be increased in accordance with an increase in the microwave power passing through the waveguide. It should be noted, however, that great stress arising from the atmospheric pressure and the pressure of the coolant will be applied to the dielectic disks, if thin dielectric disks are employed, if the distance between the disks is shortened, or if the coolant-introducing pressure is increased for an increase of the flow rate of the coolant. As a result, it becomes likely that the dielectric disks will be broken, due to the great stress.
The maximum stress .sigma. max at the central point of a dielectric disk is expressed by the following formula: ##EQU1## where k is a constant, P is the pressure difference between the two sides of a dielectric disk, D is the aperture of the dielectric disk, and the t is the thickness of the dielectric disk.
The thickness t of each dielectric disk is required to satisfy the following relation as a matching condition at an operating frequency: ##EQU2## where .lambda.d is the wavelength within the dielectric disk, and n is an integer.
As may be understood from formula (2), it is necessary either to decrease the thickness of the dielectric disks or to increase the thickness by using a large value for the integer n. However, if the thickness of the dielectric disks is increased, the passband of microwaves will be narrowed. In addition, if the dielectric disks are forced of a material which does not conduct much heat in the thickness direction, the temperature difference between the two sides of each dielectric disk will increase. (One side of each disk is in contact with coolant, while the other is not.) As a result, the thermal stress will increase, with an increase in the thickness of the dielectric disks. Therefore, it becomes likely that the dielectric disks will be easily broken.